The present invention relates to the production of sophisticated culinary utensils, such utensils having surfaces that are particularly easy to clean.
In all instances, the cleaning of metal, glass, glass-ceramic, or ceramic kitchen articles, following the cooking of food therein such that it has become "baked-on", i.e., following incomplete pyrolysis of organic substances on the surfaces of the articles, has proven to be very difficult.
Three test methods have been devised to evaluate the cleanability of those materials after food has been burned thereon, the first for use with samples of small size and the other two with full size cooking utensils.
In the first method, which has been termed the "thin burned layer test", a thin layer of a mixture consisting of an oil, albumin, sugar, and water is spread onto a sample having an approximate surface area of 16 cm.sup.2 and the sample then inserted into an oven operating at 250.degree. C. for 15 minutes. A thin layer of burned material results which is composed of partially carbonized organic substances. Typically, this layer cannot be removed from the surface of the sample by washing or even by non-abrasive mechanical action. It is necessary to immerse the sample into boiling water for several minutes in order to achieve good cleaning. This test procedure simulates the spattering of greasy substances on the sides of a cooking utensil used for baking, especially those used for roasts.
The second test method, which has been designated the "pork roast test", involves placing a pork roast into a cooking utensil and baking it in an oven. After the roast has been cooked, examination of the cooking utensil indicates that the surfaces thereof are coated with the burned-on spatterings of greasy substances which are commonly impossible to remove with a non-abrasive sponge. The utensil is placed into a home dishwasher and subjected to the normal washing cycle thereof. If the utensil is not clean, it is finished with a non-abrasive sponge.
The third test method, which has been specified as the "pork ribs baking test", contemplates placing pork ribs into a pan and baking the ribs over a flame or an electric heating element. Again, after baking, the surfaces of the pan are coated with burned-on grease spatterings which cannot be removed with a non-abrasive sponge. The same cleaning procedure as described above for the "pork roast test" is utilized.
Those three testing procedures have been complemented by a "detergent aging test" which is designed to evaluate the effect, if any, of detergent attack upon the surfaces of the material upon the cleanability of the materials. Thus, the small samples or cooking utensils are immersed for two hours at 100.degree. C. into an aqueous solution of a dishwasher detergent (MACH 1) having a concentration of one gram/liter and then washed in tap water. The materials are visually inspected to note any alterations in appearance. Thereafter, the materials are subjected to the above-described test methods to determine any change in cleanability. This aging test in detergent is repeated to the extent of six hours' immersion, which has been deemed equivalent to several years of home use in a dishwasher.
Evaluations of current, commercially-available culinary utensils prepared from glasses, glass-ceramics, ceramics, and metals utilizing the three above test methods have yielded results indicating the difficulty of cleaning those materials. For example, the required cleaning time in the "thin burned layer test" for a PYREX.RTM. brand glass article is 50 minutes, for a glass-ceramic article is 40 minutes, and for aluminum and ceramic articles is over 25 minutes. In the "pork roast test", those articles do not come fully clean after a cleaning cycle in the dishwasher followed by a finishing with a non-abrasive sponge.
To facilitate cleaning, certain metal culinary utensils have been coated with organic polymers such as polytetrafluoroethylene (PTFE) and the polysiloxanes (silicones). Evaluations of such coatings in the "thin burned layer test" demonstrated them to be anti-adherent. Contrariwise, however, when such coated utensils were subjected to the "pork roast test", it was observed that the partially pyrolyzed grease spatterings adhered rather strongly to the surfaces of the utensils. Accordingly, to secure complete cleaning, it was necessary to not only place them into the dishwasher for a normal washing cycle, but also to finish the cleaning with a non-abrasive sponge.
Although these polymer coatings are relatively easy to clean, where compared with articles of glass, glass-ceramic, ceramic, or metal, they nevertheless do present a number of disadvantages. For example, in like manner to all organic substances, they are quite easy to scratch and cannot stand high temperature heat treatments. Furthermore, in certain cases (PTFE) they yield noxious products of decomposition. Moreover, because of their porosity, certain PTFE coatings are stained by cooked foods.
French Pat. No. 2,319,594 discloses the deposition of metallic oxide deposits, such as TiO.sub.2, ZrO.sub.2, and Al.sub.2 O.sub.3, onto the surfaces of glass, glass-ceramic, and ceramic cooking vessels to improve their cleanability. Truly, those oxides can produce deposits having mechanical, chemical, and thermal resistances which are superior to those of PTFE and silicones. However, the cleaning of samples coated with those oxides according to the "thin burned layer test" requires five minutes under the best conditions. Furthermore, articles coated with those oxides which were subjected to the "pork roast test" are much more difficult to clean than articles coated with PTFE or silicones, the finishing off with a nonabrasive sponge requiring more effort.
U.S. Pat. No. 4,155,788 describes the firing of finely-divided particles of a particular shale onto the surfaces of ceramic cooking utensils. This fired-on coating is stated to render the articles readily cleanable. Because of the coating, however, the utensils cannot be transparent.